This invention relates to polycarbonates, and more particularly, to a process for the preparation of high molecular weight segmented block copolycarbonates from diphenols and tetrahalogenated diphenols.
Polycarbonates are well-known commercially available resinous materials having a variety of applications. They are typically prepared by the reaction of dihydroxy compounds with a carbonate precursor, such as phosgene, in the presence of a catalyst. Methods of direct phosgenation, interfacial condensation and transesterification, for the preparation of polycarbonates, are described in detail in Chemistry and Physics of Polycarbonates by H. Schnell, John Wiley & Co., N.Y. (1964). Polycarbonates are high temperature, high performance thermoplastic engineering polymers having a combination of good thermal and mechanical properties, especially when the polymers are prepared from one or more aromatic diols. The copolymerization of aromatic diols and tetrahalogenated aromatic diols to form copolycarbonates is commonly practiced in order to improve one or more properties of one of the homopolymers by the incorporation of the other.
Block copolycarbonates from diphenols and tetrahalogenated diphenols are known in the art. U.S. Pat. No. 3,119,787 discloses a process for producing a copolycarbonate of bisphenol A and tetrachlorobisphenol A, wherein prepolymers of each are prepared in separate interfacial polycondensation reactors, then combined in a single reactor where the prepolymers are coupled. However, phosgenation of the tetrachlorobisphenol A in the presence of a tertiary amine catalyst results in a relatively low molecular weight copolycarbonate, which is additionally difficult to wash free from salts and amine catalyst due to the formation of an excessive emulsion layer.
U.S. Pat. No. 4,413,103 discloses the preparation of halogen-containing polycarbonate resin by the copolymerization of polycarbonate oligomers having chloroformate end groups with aromatic halogencontaining polycarbonate oligomers having chloroformate end groups, wherein each of the oligomers has a molecular weight from at least 500 to 10,000. This reference asserts that the washability of the resultant copolymer is impaired when the halogen-containing oligomers have a number average molecular weight less than 1,000. A similar process having the same limitations is disclosed in U.S. Pat. No. 4,308,373.
The copolymerization of a tetrahalogenated bisphenol A and bisphenol A is disclosed in U.S. Pat. No. 3,912,687. A mixture of diphenols is phosgenated at a pH of between 7 and 9 in the presence of a tertiary amine catalyst, thereby resulting in oligomeric reaction products of the phosgene with the tetrahalogenated bisphenol A, while the more basic nonhalogenated bisphenol A remains in the bisphenolate form. Phosgenation of the mixture is then continued at a pH of greater than 13, during which time the nonhalogenated bisphenol A bisphenolates are cocondensed with the tetrahalogenated bisphenol A oligomers. Unfortunately, the disclosed process produces a relatively low molecular weight copolymer which exhibits poor washability resulting from phosgenation of the tetrahalogenated bisphenol A in the presence of a tertiary amine catalyst.